Variable speed drives utilizing a chain and sprockets involve a number of well-recognized problems, as described in U.S. Pat. No. 4,030,373, which issued June 21, 1977, and in which I am the inventor. That patent discloses a variable-speed drive suitable for use on a bicycle utilizing two belts and front and rear variable-diameter driving mechanisms. Each of these driving mechanisms included a plurality of sheave elements. Each sheave element was movable along a slot or track in a mounting plate, having serrated edges on both sides of the slot to normally retain the sheave element at a fixed radial position.
Each sheave element had releasable retaining means including a pair of locking dogs having external teeth which matched the serrations along both sides of the slot. Each sheave element also had a movable pin which extended in an axial direction from one side of the sheave element. That pin included a tapered region and was movable in an axial direction between the pair of locking dogs for spreading the dogs by a wedging action forcing their external teeth into engagement with the serrated edges on both sides of the slot. A small coil-spring normally urged the movable pin into wedging relationship between the locking dogs for spreading the dogs to cause the retention of the sheave element at a fixed position by the teeth on the dogs locking into the matching serrations on both sides of the slot.
The two belts straddled the mounting plate and engaged dual belt-carrying surfaces on the sheave elements, these dual belt-carrying surfaces being located on opposite sides of the single mounting plate.
It is my belief that the variable speed drive disclosed in that patent represented a significant technological advance over the prior "derailleur" chain and sprocket drives. However, that variable speed drive required two belts. Moreover, each sheave element was relatively complex, being an assembly with multiple moving parts, namely, including a pair of locking dogs, an axially-movable wedging pin, various rings formed on the shaft of this movable pin, an actuator head on the protruding shaft of this pin, and a coil-spring in a socket surrounding the shaft of this pin.
With two such belts straddling a mounting plate, it was difficult to keep the frictional grip equal on both belts. Therefore, one or the other of the two belts tended to pull more load than the other, thereby placing unbalanced forces on the sheave element assemblies on opposite sides in the axial direction. Also, the actuator pin head projecting axially out on one side of the sheave element assembly placed an unbalanced force on each sheave element assembly whenever radial forces were applied to the actuator pin heads for moving the sheave assemblies to new positions. Such unbalanced forces sometimes tended to jam the sheave element assemblies rather than moving them to their new positions.
The actuator pins of the sheave assemblies in these variable-diameter driving mechanisms were operated into their locking dog releasing positions by moving these pins axially. In practice, the tolerance problems of axial actuation of such pins proved difficult. A small wobble of the variable-diameter driving mechanism shown in said patent could cause some of the pins to miss their full actuation. Thus, in practice, a small wobble could cause not all of the sheave element assemblies to be actuated all of the time.
The complexities of the sheave element assemblies with their multiple parts caused them to be inordinately expensive.
In addition, the gates in the variable diameter driving mechanisms in said patent were adjusted radially by a screw-threaded rod. Therefore, these gates were adjustable by the bicycle user over a continuous range! In other words, these gates could be adjusted in position by indefinite amounts, depending upon how much the user rotated the screw-threaded rod. Since the gates were moved by indefinite amounts, the sheave element assemblies themselves became adjusted by indefinite amounts by the gates. For accommodating this (indefinite or random) positioning of the sheave element assemblies, the serrations along both edges of each slot in that variable speed drive were very fine. These serrations were made as fine as possible in order to try to accommodate the indefinite adjustment, but these serrations did have some physical size for their own strength. Thus, inevitably, there was conflict between the finite serrations and the indefinite adjustment; such conflict led to excessive wear and noise.
In order to lock the sheave element assemblies in place between such fine serrations, there was metal-to-metal locking action by the pair of locking dogs. The locking dogs were spread apart by the wedging action of the tapered portion of the axially movable pin discussed above and were held in their fully spread, locked condition by the full diameter portion of this pin. Because of these fine serrations, tolerances were critical. These critical tolerances plus the complexities of the sheave elements as discussed above caused them to be inordinately expensive.